Chamberlain also suggests that mobile-phone carriers could actually benefit from customers using Skype. "It only works over Wi-Fi, and that's a good thing," he says. "If you've got a device that you want to use indoors, then the operator doesn't have to build a network that works there." This is especially relevant for all those people whose cell-phone service drops off inside their homes. Instead of the cellular providers rushing to improve the signal coverage by adding more towers, says Chamberlain, they could just let the users supply and pay for their own Wi-Fi network to use VoIP.

A quick test of the Skype iPhone application shows the call quality to be similar to that achieved using a laptop computer. It is easy to log on, and contacts are immediately loaded on the screen.

As with the regular service, calling a non-Skype number within the U.S.--including another cell phone--costs two cents per minute. Calls received using Skype will reach a computer or iPhone simultaneously, but in order for the iPhone to ring, the application must be open and running.

While the phone quality is good as long as the Wi-Fi signal is strong, there are a number of drawbacks to using Skype on the iPhone. First, prolonged use of the Wi-Fi radio tends to suck battery power, so a Skype call will drain the battery faster than a cellular one. Second, the phone-within-a-phone experience is unintuitive and may be confusing to some users. Ideally, Skype would run in the background and incoming calls from Skype or the cellular network would look the same. (The next upgrade of iPhone software may allow Skype calls to be pushed through even when the application isn't actively running). And third, the iPhone application doesn't have all the features that the desktop version of Skype does, like text messaging and video chat. The position of the iPhone's camera, on the opposite side of the screen, makes two-way video chatting unfeasible.

Skype for the iPhone and iPod Touch is free, although the iPod requires a microphone add-on. In May, Skype plans to release an application for the BlackBerry as well.